The present invention relates to a system and method for controlling the air-fuel ratio for an internal combustion engine emission control system with a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximately equal to the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburetor. The control system operates to control the air-fuel ratio of the mixture to be supplied to the engine depending upon the signal from the oxygen sensor. However, if misfiring occurs as is experienced during rapid deceleration of the vehicle, a large amount of oxygen remains in the exhaust gases, which is the same condition when a lean mixture is induced and burned in the engine. Accordingly, the oxygen sensor generates a signal indicating a lean mixture, so that the control system operates to vary the mixture to provide a rich air-fuel ratio. Therefore, even if the actually induced mixture is a rich or a proper air-fuel ratio, the mixture is further enriched by the control of the system to an excessively rich air-fuel ratio. Thus, fuel consumption of the engine increases and unburned constituents of the exhaust gases increase.